Dear Colleagues,

The global human population increased from one billion in 1800 to 7.6 billion in 2017. During this period, the population has become more affluent and there is the generation of a tremendous amount of waste. On average, every person in the United States generates and discards rubbish equivalent of his/her body weight each month (Hoornweg et al., 2013). This may also be true for citizens of other countries. Certainly, rapid waste generation and accumulation have had a significant effect on the global potable water reserve and wildlife welfare. I contend that there needs to be a great investment into developing environmentally friendly technologies that: Facilitate decreases in waste transfer to landfills and incinerators; promote water recycling; prevent contamination of underground water; and mitigate some of the damage already caused by excessive waste generation and poor disposal choices. Hence, from my perspective there is an urgent need to re-evaluate the role of anaerobic digestion (AD) processing of wastes to address these important societal issues.

Notably, AD occurs naturally in the absence of oxygen during which a diverse population of microorganisms degrade putrescible materials such as sewage sludge, organic farm wastes, food and restaurant wastes, organic industrial and commercial wastes, and municipal solid wastes, into biogas (methane and CO₂) and digestate (solid–liquid effluent mixture). Indeed, AD processing dates back to 17th century when Belgian scientist and philosopher, Mr. Jan Baptista van Helmont, observed that flammable gases could be emitted from putrefying organic materials. It was not until 19th century (1808) that Sir Humphry Davy determined that methane in biogas produced during AD of cattle manure was responsible for this flammability.

At present, the term ‘anaerobic digestion’ refers to both a natural process and a developed technology. Although it is an old technology, the AD processing of wastes is gaining traction recently as an eco-friendly waste treatment and recycling option to produce energy and fertilizer (compost) from organic wastes along with reducing odor emissions and improving air quality. The AD process is typically divided into four steps (1-hydrolysis, 2-acidogenesis, 3-acetogenesis, and 4-methanogenesis), and with each step there are problems and challenges associated with the operation of an AD plant, in addition to technical problems such as inadequate mixing, clogged pipes, floating layers and bulky sediments in the digester. Additionally, problems associated with heterogeneity of feedstock, operating costs, disposal of digestate, odor emissions, and digester failures are some of the challenges that occur when operating AD plants. Thus, in this Special Issue, we invite authors to submit original research and review articles on all aspects of AD research including but not limited to biochemistry, ecology and physiology of AD microorganisms that enhance our understanding of the AD process, expand on current state of the art technologies of the AD process, and problems associated with AD and mitigation strategies.

References

Hoornweg D, Bhada-Tata P, Kennedy C (2013) Environment: Waste production must peak this century. Nature 502: 615–617

Prof. Dr. Thaddeus Ezeji
Guest Editor

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• Municipal solid waste
• Wastewater treatment
• Anaerobic digester
• Anaerobic digestion
• Methane
• Gas fermentation
• Biogas
• Methanogenesis
• Biomethanization
• Acetogenesis
• Biogasification
• Hydrolysis
• Single-stage digester
• Two-stage digester
• Upflow anaerobic sludge blanket
• Downflow Upflow Hybrid Reactor
• Upflow Fixed Film Loop Reactor
• Downflow Stationary Fixed Film Reactor
• Baffled anaerobic digester
• Fluidized bed reactor
• Biological oxygen demand (BOD)
• Chemical oxygen demand (COD)